Polylactic acids are more excellent in heat resistance with higher glass transition temperatures (Tg) than other biodegradable resins, but the heat resistance of the polylactic acids in a temperature range higher than Tg is not necessarily high. Since the polylactic acids have lower crystallization rates, the molding cycle for injection molding should be increased. Further, the polylactic acids have lower melt viscosities, so that molding conditions are significantly limited. Therefore, the molding productivity is relatively low.
For improvement of the heat resistance and the productivity, the inventors of the present invention previously proposed in JP-A-2003-128901 and JP-A-2003-238789 that a biodegradable polyester is crosslinked by addition of a (meth)acrylate compound or a polyvalent isocyanate compound. Further, the inventors proposed in JP-A-2003-147182 that a layered silicate is additionally used.
On the other hand, JP-A-2001-261797 discloses a technique for improving the heat resistance and the hydrolysis resistance by blocking terminal carboxyl groups of a polylactic acid by a specific carbodiimide compound.
The heat resistance and moldability of the polylactic acids are improved by the crosslinking and the addition of the layered silicate. However, the polylactic acids fail to maintain their physical properties due to hydrolysis during prolonged storage or during use under severely humid and hot conditions. Therefore, the practicality of the polylactic acids is not sufficient under such conditions.
Further, the polylactic acid with its terminal groups blocked by the carbodiimide compound as disclosed in JP-A-2001-261797 is not suitable for production of injection molded articles, foamed articles and blow-molded articles.